I. Field of the Invention
The invention relates to liquid crystal displays (LCDs). More specifically, the invention describes a method and apparatus for automatically determining a pixel clock.
II. Description of the Related Art
Digital display devices generally include a display screen including a number of horizontal lines. The number of horizontal and vertical lines defines the resolution of the corresponding digital display device. Resolutions of typical screens available in the market place include 640×480, 1024×768 etc. At least for the desk-top and lap-top applications, there is a demand for increasingly bigger size display screens. Accordingly, the number of horizontal display lines and the number of pixels within each horizontal line has also been generally increasing.
In order to display a source image on a display screen, each source image is transmitted as a sequence of frames each of which includes a number of horizontal scan lines. Typically, a time reference signal is provided in order to divide the analog signal into horizontal scan lines and frames. In the VGA/SVGA environments, for example, the reference signals include a VSYNC signal and an HSYNC signal where the VSYNC signal indicates the beginning of a frame and the HSYNC signal indicates the beginning of a next source scan line. Therefore, in order to display a source image, the source image is divided into a number of points and each point is displayed on a pixel in such a way that point can be represented as a pixel data element. Display signals for each pixel on the display may be generated using the corresponding display data element.
However, in some cases, the source image may be received in the form of an analog signal. Thus, the analog data must be converted into pixel data for display on a digital display screen. In order to convert the source image received in analog signal form to pixel data suitable for display on a digital display device, each horizontal scan line must be converted to a number of pixel data. For such a conversion, each horizontal scan line of analog data is sampled a predetermined number of times (HTOTAL) using a sampling clock signal (i.e., pixel clock). That is, the horizontal scan line is usually sampled during each cycle of the sampling clock. Accordingly, the sampling clock is designed to have a frequency such that the display portion of each horizontal scan line is sampled a desired number of times (HTOTAL) that corresponds to the number of pixels on each horizontal display line of the display screen.
In general, a digital display unit needs to sample a received analog display signal to recover the pixel data elements from which the display signal was generated. For accurate recovery, the number of samples taken in each horizontal line needs to equal HTOTAL. If the number of samples taken is not equal to HTOTAL, the sampling may be inaccurate and resulting in any number and type of display artifacts (such as moire patterns).
Therefore what is desired is an efficient method and apparatus for determining a pixel clock of an analog video signal suitable for display on a fixed position pixel display such as an LCD.